One of conventional seat belt retractors has a spool and a torsion bar inserted into an inner hole of the spool. One end of the torsion bar is fixed to the spool and the other end of the torsion bar is engageable with a retractor frame by a clutch mechanism. In an emergency such as collision, the other end of the torsion bar is locked by the clutch mechanism to the frame. As the one end of the torsion bar is fixed to the spool, the torsion bar is twisted by the force which is exerted in the webbing-unwinding direction to the spool, resulting in the gradual rotation of the spool and the gradual unwinding of the webbing. In this way, a part of the kinetic energy delivered to the body of an occupant is absorbed by the twisting of the torsion bar, and the impact is alleviated.
One of examples of seat belt retractors having such a torsion bar (JUB 61-11085) is shown in FIGS. 5 through 8.
In FIG. 5 and FIG. 6 a retractor body 1 is fixed to the vehicle by inserting a bolt into a hole 11 formed in the frame thereof.
A pair of side walls 12, 13 are formed in the frame 1 and a spool 2 is journaled rotatably on the walls. The spool 2 is formed in a cylindrical shape to have an inner hole in which a torsion bar 3 is disposed.
The torsion bar 3 comprises a bar-like torsional portion 31, a joint 32 formed on one end of the torsional portion 31 for connecting to the spool 2, a threaded portion 33 which has a larger diameter than the torsional portion 31 and is provided with a thread 331 formed in its outer surface, and a gear portion 34 on the end of the threaded portion 33. The gear portion 34 comprises a flange-like circular plate 341 and saw-teeth-like one way ratchet teeth 342 on the outer periphery of the circular plate 341. The torsional portion 31 is arranged in the inner hole of the spool 2 coaxially with it. The joint 32 having a rectangular head is fitted into a rectangular hole formed in the wall 21 at one end of the spool 2 Accordingly, the joint 32 which is located at the one end of the torsion bar 3 rotates together with the spool 2. A ring 35 is fitted on the outer periphery of the threaded portion 33 to confront the inner surface of the spool 2 in such a manner that it can slide against the spool 2.
Formed on one end of the spool 2 is a flange 22, one face of which is sidably disposed to the side wall 13 of the retractor body 1 and the other face confronting the plate 341 of the gear portion 34 with a certain space between them has a plurality of projections 23, which have rectangular cross sections.
A ring-like stopper member 4 having female thread on its inner periphery is screwed on the threaded potion 33 of the torsion bar 3 in such a manner that it can easily rotate. Holes 41 are formed in the stopper member 4, at the positions corresponding to the projections 23, and the projections 23 are inserted loosely into the holes 41.
One end of a webbing S is connected to the torsional portion 31 of the torsion bar 3. The webbing S is wound on the spool 2 then introduced out of the spool, and the unwound end is fixed to an anchor or a buckle (not shown). A casing 14 is arranged on the side of the side wall 12 of the retractor body 1. The casing 14 has a spiral spring (not shown) which biases lightly the spool 2 in the webbing-winding direction (the direction indicated by an arrow Y.sub.1 in FIG. 6). The webbing S can be unwound in accordance with the posture of the occupant (the direction indicated by an arrow Y.sub.2 in FIG. 6).
Fixed on the lower ends of both side walls 12, 13 is a bracket 15 movably supporting the pendulum 5. A ratchet 6 is supported on the upper end of the pendulum 5 in such a manner that the ratchet is inclined to be lifted by the movement of the pendulum 5. The lifted ratchet 6 engages its one end with the confronting tooth of the one way ratchet teeth 342 of the torsion bar 3 to lock the rotation of one end of the torsion bar 3.
In the seat belt retractor mentioned above, the torsion bar 3 is usually not locked by the ratchet 6, and the torsion bar 3 and the spool 2 are rotatable, thereby allowing the unwinding and winding of the webbing S in accordance with the posture of the occupant.
When the vehicle comes in collision, the pendulum 5 sways to lift the ratchet 6, as shown in FIG. 7 and FIG. 8. The ratchet 6 therefore engages one of the ratchet teeth 342 of the gear portion 34 of the torsion bar 3 to lock the rotation in the webbing-unwinding direction (the direction indicated by the arrow Y.sub.2 in FIG. 8).
On the other hand, webbing S is tensioned by the movement of the occupant body, causing the rotation of the spool 2 in the webbing-unwinding direction. In this instant, if the rotational force exceeds the stress limit of the torsional portion 31 of the torsion bar 3, the spool 2 rotates with twisting the torsional portion 31. In this way, the impact energy to the occupant is absorbed.
When the spool 2 rotates, the stopper member 4 which is engaged with the projections 23 arranged in one end of the spool 2 also rotates. As the stopper member 4 is screwed rotatably onto the threaded portion 33 of the torsion bar 3, and is fitted loosely with the projections 23 of the spool 2, the stopper member 4 is driven toward the plate 341 which has teeth formed in the outer periphery. After the spool 2 has rotated and the torsional portion 31 of the torsion bar 3 has been twisted for a predetermined angle within which it does not break, the stopper member 4 comes in contact with the plate 341 and thus, the rotation of the spool 2 is stopped. Consequently, the torsion bar 3 is prevented from breaking even if the excess load is exerted from the occupant to the webbing S.
However, it is difficult to fix the end of the webbing firmly onto the spool which has torsion bar inserted in its inner hole as mentioned above. In addition, the JUB 61-11085 has no description as to how the webbing S is fixed onto the spool 2.
In the example shown in FIG. 5 through FIG. 8, the stopper member 4 is arranged outside of the flange 22. Accordingly, the seat belt retractor protrudes toward the side of the frame for the total length of the thickness of the stopper member 4 and its stroke along the projections 23.